caisson

caisson (kāˈsən, –sŏn) [key] [Fr., = big box], in engineering, a chamber, usually of steel but sometimes of wood or reinforced concrete, used in the construction of foundations or piers in or near a body of water. There are several types. The open caisson is a cylinder or box, open at the top and bottom, of size and shape to suit the projected foundation and with a cutting edge around the bottom. It is sunk by its own weight and by excavation, then filled with concrete. Pneumatic caissons are usually employed in riverbed work or where quicksand is present. In this type the cylinder or box has an airtight bulkhead high enough above the cutting edge to permit men to work underneath it. The air in the chamber beneath the bulkhead is kept under pressure great enough to prevent the entrance of water, while shafts through the bulkhead permit the passage of men, equipment, and excavated material between the bottom and the surface. At the top of each shaft is an air lock to permit communication with the outside without altering the air pressure in the working chamber. As the working chamber moves down, the caisson above the bulkhead and about the shafts is filled with concrete, and when a sufficient depth or bedrock is reached, the working chamber itself is filled, so that there is a solid block of concrete from base to top. Workers leaving a pneumatic caisson after hours of labor under high pressure are given special decompression treatment to accustom them to the lower atmospheric pressure and thus to prevent caisson disease (see decompression sickness). A type of caisson often called a camel is used to raise sunken vessels. It consists of a cylinder filled with water, which is sunk, attached to the vessel, and emptied by pump or compressed air, so that its buoyancy can assist in raising the vessel. Caissons are also sometimes used for closing the entrance to dry docks or as a substitute for gates in canal locks.